Stereolithography is a technique that belongs to the family of rapid prototyping and manufacturing techniques. Generally these techniques allow the fabrication of three-dimensional objects directly starting from computer generated CAD files. In stereolithography the parts are first divided into a stack of successive layers that represents the three-dimensional object as closely as possible. The object itself is then constructed on the machines according to the computer generated layers. First a layer of resin is deposited over the entire building area. Secondly the sections of the building area that are part of the object to be constructed are illuminated, thereby causing the resin on the illuminated areas to polymerize and harden. Upon completion of this layer a new layer of resin is deposited and the process is repeated until the object is completely finished. Then the solidified object can be removed from the resin and processed further. Stereolithography therefore represents a fast way to make complex or simple parts without tooling. As this technology depends on using a computer to generate its cross-sectional patterns, there is a natural data link to CAD/CAM.
However, objects built using stereolithography have a tendency to distort from their CAD designed dimensions. Differential shrinkage is one of the most disturbing flaws often detected in many parts fabricated by stereolithograpy. It is caused by the transition of the resin to the solidified polymer. This transition is accompanied by a shrinking of the material. Moreover certain geometries or sections in a part are more susceptible than others. Especially large flat surfaces are susceptible to shrinkage. Because not all sections show the same amount of shrinkage, or differential shrinkage, the parts are deformed and no longer correspond to their original CAD representation. For instance a large flat surface connecting to thin upright structures can lead to significant part deformation. Therefore, there is a need for a technique to reduce vertical distortions.
In the art there have been several efforts to try and avoid this type of deformation by changing the scanning techniques, by changing the pattern (e.g. the use of tightly packed hexagonal tiles in the pattern), the use of several subsequent exposures of the same lamina or by providing intermediate solidification steps for intermediate lamina or layers. Allison et al. (U.S. Pat. No. 5,256,340) disclose various scanning techniques for forming objects including various techniques for reducing object distortion. Vinson et al. (U.S. Pat. No. 5,238,639) disclose a technique for minimizing part curl by curing a balancing layer in relation to a balanced layer such that reverse curl of the balanced layer caused by the balancing layer offsets or negates normal curl of the balanced layer caused by the balancing layer. Hull et al. (U.S. Pat. No. 5,273,691) describe an improved stereolithography system with the aim of reducing curl, a phenomenon closely related to differential shrinkage. Different scanning techniques are described therein to accomplish this. Manners et al. (U.S. Pat. No. 5,965,079) describe a stereolithography method wherein a pattern of tightly packed hexagonal tiles are drawn with the aim of reducing shrinkage. Guertin et al. (U.S. Pat. No. 6,399,010) describe the use of a time delay between a first and second exposure of portions of a lamina. The delay may be determined by a clock, or alternatively, the time delay may be considered to have lapsed upon certain physical conditions being met. Manners et al. (U.S. Pat. No. 6,649,113) describe employing an intermediate solidification step for intermediate lamina or layers for reducing the amount of resin that will shrink upon solidification. Nguyen et al. (U.S. Pat. No. 6,699,424) describe a method that solidifies the main part area, delays or pauses for a desired period of time to permit shrinkage to occur, and then, in multiple drawings of the main part borders, solidifies the borders from the portion closest to the main part outwardly to the portion farthest from the main part.
Several of these techniques are based on a non-standard way of illuminating the areas to be polymerized. This however requires a complex redesign of the slicing and hatching software, leading to visible markings on the outside of the parts and negatively influencing quality of the part. Moreover these techniques also result in a slower construction time. Other prior art techniques rely on employing waiting times at moments in the building process where this is not normally done in the building process. This therefore requires complex redesign of both building and preparation software, leading to a slower production time and reduction of the quality of the produced parts.
Accordingly, there is a need in the art for improved stereolithography techniques. It is accordingly one of the aims of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.